This invention concerns fluid-bed combustion reactors and a method for the operation of a fluid-bed combustion reactor. The invention further concerns a fluid-bed cooler for particulate material.
Fluid-bed systems are used in a number of processes, wherein a good contact between solid particulate material and gas is desired. Examples are heat exchange, reactions with heterogeneous catalysts and reactions directly between solid matter and gases. The fluid-bed principle may briefly be explained in that the solid particulates are affected by a fluidization gas introduced from below, it being within certain constraints possible hereby to suspend the particles within a body of particulate materials and keep them suspended, even though the gas flow velocity does not need to rise to a level where single particles except for the very smallest ones would be entrained and carried away by the gas flow. Under such conditions the individual particles are freely movable, but the body of the particulate material will exhibit an upper surface, i.e. it behaves like a liquid from which the name fluid-bed. Hereby, obviously a very large area of contact between the solid particulates and the applied gas is achieved.
Recently fluid-bed systems have acquired a special interest in connection with applications related to combustion systems for solid fuels. Important advantages are that fluid bed systems may operate on various types of fuel and that an extremely good heat transfer from the combustion may be obtained. The body of particles within such systems may comprise inert particles such as sand, into which a minor proportion of fuel is added. The inert particles are heated by the combustion and circulate within the fluid-bed contacting suitable heat exchanger surfaces to transfer heat hereto. Heat transfer by radiation or by gas convection to fixed heat exchanger surfaces which is usual with other combustion systems will thus to some extent be replaced by heat transfer through physical transport of particles, whereby extended contact areas and heat exchange by direct contact between solid matter is obtained, whereby the heat exchange coefficient (number of watts exchanged related to m.sup.2 's of surface area and related to degrees of temperature difference) is higher than that achieved by the contact between gas and fixed surface.
Fluid-bed combustion systems allow a closer control of combustion parameters and make it possible to clean the exhaust gas for certain undesirable materials as reactants may simply be intermixed into the bed material, making it possible to achieve a combustion which in several respects is more environmentally acceptable than it is possible with other combustion systems. However, besides these advantages there are also certain difficulties connected to fluid-bed reactors, among which may be noted that they are substantially more complicated than other combustion systems by requiring the controlled introduction of fluidization gas, and by requiring extended start-up periods, e.g. of the magnitude of 3 to 10 hours, due to the substantial amount of solid material to be heated. Furthermore, it is difficult to operate them completely satisfactory by partial load, and adjustments of the load can only be carried out slowly.
Fluid-bed combustion systems are traditionally classified by the mean velocity of fluidization gas upwards through the fluid-bed, several variants occurring operating at various gas velocities within a range that may be generally described by the limits designated slow beds and fast beds, respectively.
Slow beds are characterized by a fluidization velocity typically within the range 1 through 3 m/second, this velocity having lower limits defined by the requirement for oxygen to the combustion and by the requirement for a minimum gas velocity in order to fluidize the particles. The density within the body of particles will be relatively high and the bed must be relatively shallow in order to keep the gas pressure necessary for fluidization within reasonable limits. However, hereby the dwell time for fuel particles and for the gas within the bed becomes too short to ensure a complete combustion, slow beds therefore exhibiting not quite satisfactory combustion efficiency and little possibility for cleaning of the exhaust gas.
Fast beds are characterized by a fluidization velocity within the range of approximately 3 through 12 m/second, whereby a substantial portion of bed particles are entrained by elutriation with the fluidization gas and must be recirculated back to the bed. They are also designated circulating beds and do not exhibit any well-defined bed surface. They may provide a superior combustion and superior exhaust gas cleaning than slow beds, but have the disadvantage of requiring extended systems to separate bed particles from the exhaust gas and recirculate the particles. Another disadvantage related to fast beds is that the heat exchange coefficient between said particles and heat transfer surfaces is inferior at the higher velocities as compared to the velocities typical in the slow beds.
In the past several attempts have been made to devise designs obtaining the consolidated advantages of the slow beds and of the fast beds.
U.S. Pat. No. 4,111,158 to Reh et al. e.g. discloses a fluid-bed reactor with a fast bed, in which combustion takes place, a cyclone to separate the bed particles from the exhaust gas and a fluid-bed cooler, wherein the separated particles are passed through a secondary fluid-bed of the slow type, wherein the particles exchange and dissipate their heat to heat transfer surfaces. The system described is very complicated and extensive, which is considered extremely undesirable, keeping in mind that all conducts and transportation systems must be designed to withstand combustion at temperatures of the magnitude of 800.degree. C.
U.S. Pat. No. 4,788,919 to Holm et al. discloses a more compact solution comprising a central combustion bed with gas inlets at the bottom and optionally with secondary gas inlets located hereabove, from which particles are elutriated and carried up into a top chamber, and with a secondary fluid-bed or a fluid-bed cooler arranged annularly around the central fluid-bed at a level above the central fluid bed so that the particles transported up into the top chamber may drop down into this secondary fluid-bed. In the secondary annular fluid bed, which is a slow bed, particles may dissipate their heat to heat transfer surfaces and the particles may thereafter by means of gravity flow back to return to the central primary fluid bed.
U.S. Pat. No. 4,594,967 to Wolowodiuk discloses a fluid-bed combustion reactor with a primary bed, a top chamber and a fluid-bed particle cooler arranged in such a way that particles entrained with the gas flow from the primary bed may enter the top chamber and drop down to the particulate cooler, wherein the particles pass serpentine tubes and are cooled. From the cooler the particles pass a valve means down to a storage chamber and from the bottom of the storage chamber the particles may pass another valve means to return to the primary fluid-bed. This design is relatively compact, but no possibility is disclosed for varying the relation between the various areas of cooling sections apart from a possibility for partly emptying the particle cooler by conveying particles down into the storage chamber so that a portion of the cooling tubes in the particle cooler will no longer be covered by particles. However, a such method of operation must be considered extremely disadvantageous as the particles serve the purpose of protecting the tubes against the corrosive effects of the exhaust gases and as any portion of tube situated just above the upper surface of the fluidized particles will be subjected to abrasive wear by particles thrown upwards from the fluid bed and hitting the tube with some velocity. The document includes no disclosure regarding the design of the valves for the flow of particles, mentioning only that they may be activated selectively. Thus, no facility for the continuous control or facility for obtaining a constant controlled flow of particles downwards through the particle cooler and returning to the reactor is shown.
The provision of a separate fluid-bed particle cooler is a considerable improvement to fluid bed combustion systems, however, substantial problems remain, which have as yet not been solved quite satisfactorily. The heat transfer systems briefly mentioned in the above patents will e.g. for power generator purposes normally comprise a water preheater, also designated an economizer, an evaporator, in which the water is evaporated, and a super-heater, in which steam is super-heated. These heat transfer systems operate at different temperatures and must therefore be arranged paying regard to heat energy transfer requirements and applicable temperatures. Another factor that must also be taken into account is that the heat transfer systems also serve the purpose of protecting the constructional elements against the elevated temperatures. In practical fluid-bed combustion systems the greater part of the walls must therefore be provided with heat transfer systems. The economizer, which operates at a relatively low temperature, is preferably arranged in the exhaust gas duct after other heat exchangers. The super-heater operating at the highest temperature, e.g. 500.degree. to 530.degree. C., is conveniently arranged with a greater portion within the fluid bed, where the good heat transfer coefficient for the particles and the heat transfer surfaces make possible the heating to the high temperatures and with a smaller portion in the exhaust gas duct. It is noted that by the greater and smaller portion is understood portions with greater and smaller heat power transfer rather than geometrically greater and smaller portions. Within the fluid-bed particle cooler the super-heater may also to some extent be protected against corrosion and erosion, which is a critical factor at the elevated temperatures.
Evaporator tubes are conveniently utilized for cooling the walls, but since typically the area of evaporator surfaces needed exceeds what can be integrated into the walls, further sections of evaporator tubes are arranged within the fluid-bed cooler or in the exhaust gas duct before the economizer, or sections of evaporator tubes may be arranged in all of these places. The areas of the various heat transfer surfaces are naturally fixed once the reactor has been built.
However, the optimal relation between the areas of the various heat transfer surfaces depend upon the type of fuel used. E.g. fuels developing a relatively large proportion of water or steam in the exhaust gas ideally need a relatively smaller evaporator surface area than it is the case by combustion of coal. Fuels developing a larger proportion of water or steam could e.g. be fuels actually containing water such as particles of coal suspended in water or fuels which due to a content of hydrogen develop water by the combustion such as is the case with straw or wood. In case a plant designed for the optimal combustion of coal is to burn straw, the water-flow through the heat transfer surfaces must be reduced, but hereby the temperature in the evaporator sections may rise unacceptably. Similar problems may arise by partial load. To operate at partial load the air flow is reduced while the temperature within the reactor is kept substantially unchanged. The heat radiated onto the reactor walls which is ultimately transferred into the evaporator tubes arranged within the walls is therefore not reduced very much and the temperatures within the evaporator tubes may therefore tend to increase by the reduced water flow. The opposite problem might however, depending upon the particular circumstances, also occur, i.e. the temperature of the super-heater tubes could increase too much by a load reduction, in particular in case the heat transfer surfaces are arranged partly in the exhaust gas duct and partly within the fluid-bed cooler. By partial loads the gas-flow for fluidization is reduced, but hereby the heat transfer from the exhaust gases drops much more than the heat transfer within the fluid-bed. As mentioned above the super-heater surfaces are often arranged for the greater portion within the fluid-bed, and in case a substantial portion of the evaporator surfaces is arranged in the exhaust gas flow the super-heater temperature may rise too much due to the reduction of the water-flow. It is here noted that the temperature within the fluid-bed and therefore within the combustion chamber should be kept within a narrow range for satisfactory operation of the fluid-beds at full load as well as at partial loads. The strategy practically adhered to in the prior art is the adding of water at suitable points between sections of the evaporator tubes and before the super-heater in order to ensure that the tube temperature is kept within safe limits, which, however, does not provide the best economy of the system.
A further reason for inferior efficiency by systems of the prior art operating at partial load is that the amount of particulate matter in the reactor may not be optimal. By partial load the fluidization velocity will be reduced and the density of the bed will therefore be increased. In order to obtain a predetermined level of the beds the amount of particulate matter must therefore also be altered.